Centralize boiler-plate model instantiation
Rename cpymad.madx, cpymad.model constructor functions

Renamed to:

- start_madx
- load_model

The new names more idiomatically describe the actions.

The intention is to remove name clashes with the corresponding
submodules, as these had some undesirable effects with the import system.
Merge pull request #57 from coldfix/fix-package-name-clash

name clash of package variables and module names